fix: do not create spans when confluent-kafka.poll() response is empty

[CagriYonca]

This PR solves the confluent-kafka bug below:

Traceback (most recent call last):
  File "/opt/instana/instrumentation/python/instana/instrumentation/kafka/confluent_kafka_python.py", line 254, in trace_kafka_poll
    create_span("poll", res.topic(), res.headers())
                        ^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'topic'

Note: This issue doesn't happen in the Kafka-Python package since the poll function there works differently.

[pvital]

It looks good to me.